If you're trapped under someone's full mount, usually the only chance you have of escape is rolling over. Call it cowardly all you want, it's a valid technique that can lead to a transition where otherwise the fight would have been over. It's also a very instinctual move to avoid taking more undefended strikes to the face, so yeah some guys will roll over just to avoid taking more shots. It is t a desperation move? Absolutely. It's very risky and often will lead to a RNC, but not always. There are plenty of examples of escapes from the position.
I don't understand why people equate tapping to "courage" of any kind. It's high school shit for 35 year-old Affliction T shirt wearing bums.